The antenna requirements of a cellular or cordless telephone handset are primarily that it should be compact and omnidirectional. For a handset operating within the frequency range of 800 MHz to 2 GHz the antenna is typically an extendable rod having a length approximately equivalent to the a quarter wavelength when extended, or a helical wire having several turns. The antenna is usually mounted partially within the handset unit and partly projecting from the end of the unit adjacent the earphone. One difficulty with radio telephone handsets is the perceived health hazard associated with prolonged irradiation of the user's head by the intense electric and magnetic fields generated close to the antenna. Typically, 90 per cent of the radiated power is absorbed by the head, particularly by the blood-rich parts such as the ears and lips. Absorption of radiation by the head can also lead to radiation inefficiency and consequent reduction of the operating range of the handset, depending on the orientation of the handset and user with respect to the nearest base station.
Other antennas for operation within the frequency range (800 MHz to 2 GHz) employed by cellular telephones include the so-called Inverted-F antenna. This has two resonant patches, one spaced above the other. However, the antenna is mechanically bulky.
In co-pending U.S. application Ser. No. 08/351,631 there is disclosed a miniature satellite navigation antenna having elements formed by four helical conductive tracks on the outer surface of a ceramic rod made of a material with a relative dielectric constant of 36. The helical elements are arranged primarily for receiving circularly polarised signals.
One of the objects of the present invention to provide an improved radio telephone handset antenna which results in reduced radiation into the user's head.